LONDON -- Police said Thursday the victim of a fatal knife attack in central London overnight was an American woman in her 60s and ruled out terrorism as the likely motive.
London Metropolitan Police Assistant Commissioner Mark Rowley said authorities' main line of inquiry was around mental health issues. Officials briefed on the investigation had earlier said they believed the stabbing was a terror attack based on initial evidence.
Police said it had arrested a 19-year-old man on suspicion of murder. The suspect is a Norwegian national of Somali origin, Rowley said, and "so far we have found no evidence of radicalization."
"Whilst the investigation is not yet complete -- all of the work that we have done so far, increasingly points to this tragic incident as having been triggered by mental health issues. At this time we believe this was a spontaneous attack and the victims were selected at random," Rowley said.
"We now know more about the victims -- an Australian, American, Israeli and British -- we have spoken to the relevant embassies."
Police said two other women and three men were injured. An earlier police statement said two remained hospitalized.
Police presence beefed up
Londoners woke up Thursday to a heavier police presence on their streets after the attack in the heart of the city late Wednesday night.
Rowley said it was "to provide reassurance and safety. We ask the public to remain calm, vigilant and alert."
The attack happened in Russell Square, an area popular with tourists near museums and upscale hotels and homes.
The stabbing came hours after authorities announced London would increase its presence of armed police by 600 officers following recent terror attacks across Europe -- part of what it called Operation Hercules.
Police were called after 10:30 p.m. Wednesday after reports of a man "seen in possession of a knife injuring people," authorities said in a statement.
Officers used a stun gun on the suspect, and he was arrested less than 10 minutes later, authorities said.
He was treated in the hospital and is in police custody at a south London station, police said.
Dying in their arms
A 40-year-old Brazilian man living in London said a Spanish family stopped and asked him to call an ambulance.
"I thought someone had collapsed or passed out, that was my first impression," said Fernando, who did not want to give his last name.
He said one of those family members was holding the dying woman and helping and reassuring her. She had been stabbed in the back, Fernando said, adding that he was in shock.
"It was not nice to see someone dying," he said.
'We were lucky'
Emma Mountford, 41, a British woman from Derbyshire, said she feels "helpless and sad" after the attack. She said she had noticed an increased armed police presence in the city.
"The scariest things about these attacks are they completely out of the blue, carried out by ordinary people," Mountford said outside the British Museum near the scene of the attack.
"We're were saying to (the) taxi driver, you can't let it stop you from doing things, otherwise you'd be terrified to leave the house. I just make sure the children stay close and always be on the (lookout) for anything odd or out of the ordinary."
Philippa Baglee said she saw the aftermath of the attack.
"I came out of the Atrium (bar) to have a cigarette, and I saw people standing over there looking at the ground. I could see someone laying down and a guy with a motorbike helmet balanced on his head just walking up and down," she said.
Janet Pavely, who was with Baglee, said people had to be "very self-aware" when out in public now.
"It could happen anywhere. I don't think it would stop us from coming back. We were lucky last night. We were going to walk, we took a cab instead. Who knows?" she said.
Police were lifting cordons around the crime scene by late Thursday morning. Workers sprayed and scrubbed the sidewalks clean as people began to leave flowers at the site.
London mayor calls for calm
London Mayor Sadiq Khan urged residents to remain "calm and vigilant" after the attack.
"The safety of all Londoners is my number one priority and my heart goes out to the victims of the incident in Russell Square and their loved ones," Khan said in a statement from his press office.
Khan asked Londoners to report anything suspicious to the police.
"We all have a vital role to play as eyes and ears for our police and security services and in helping to ensure London is protected," he said.
Russell Square, in the Bloomsbury district of west-central London, is a busy passing point to get to sites, including the British Museum and the main buildings of the University of London.
The park isn't particularly bustling at night compared with other nearby neighborhoods that are home to more restaurants and bars.
In the July 7, 2005, terror attacks on London, the most devastating of the four bombs hit the Underground subway line running close to Russell Square.
In December, Muhyadin Mire, 29, was charged with attempted murder after authorities say he carried out an unprovoked knife attack in a subway station in London's Leytonstone suburb.